Recently, a multiple input multiple output (MIMO) system receives much attention to maximize the performance and communication capacity of a wireless communication system. Emerging from the use of a single transmission antenna and a single reception antenna as it has been to date, the MIMO technique employs multiple transmission antennas and multiple reception antennas to enhance a data transmission/reception efficiency. The MIMO system, also called a multiple antenna system, is an application of a technique that collects data fragments received via several antennas for completion, rather than relying on a single antenna path, to receive a single overall message. As a result, a data rate at a particular range can be improved or a system range can be increased over a particular data rate.
The MIMO technique includes transmit diversity, spatial multiplexing, beamforming, and the like. The transmit diversity is a technique that transmits the same data from multiple transmission antennas to thus enhance a transmission reliability. Spatial multiplexing is a technique that simultaneously transmits different data from multiple transmission antennas to thus transmit high speed data without increasing a bandwidth of a system. Beamforming is used to increase a signal to interference plus noise ratio (SINR) of a signal by adding a weight value according to a channel state at multiple antennas. In this case, the weight value may be represented by a weight vector or a weight matrix, and it is called a precoding vector or a precoding matrix.
Spatial multiplexing includes spatial multiplexing for a single user and spatial multiplexing for multiple users. The spatial multiplexing for a single user is called a single user MIMO (SU-MIMO), and the spatial multiplexing for multiple users is called spatial division multiple access (SDMA) or multi-user MIMO (MU-MIMO). The capacity of a MIMO channel increases in proportion to the number of antennas. The MIMO channel may be disintegrated into independent channels. If the number of transmission antennas is Nt and the number of reception antennas is Nr, the number of independent channels Ni is Ni≦min {Nt, Nr}. Each independent channel may be a spatial layer. A rank is the number of non-zero eigenvalue of the MIMO channel, which may be defined as the number of spatial streams that can be multiplexed.
The number of transmission antennas of a terminal (i.e., user equipment (UE)) supporting the MIMO technique may vary depending on the performance of the UE. A base station (BS) must transmit data in consideration of a UE that can support a smaller number of transmission antennas than a maximum number of transmission antennas as well as a UE that supports the maximum number of transmission antennas. For example, if the BS has eight transmission antennas (Tx), the BS should employ an 8Tx MIMO technique to transmit data to a UE that can receive the data via eight transmission antennas and employ a 4Tx MIMO technique to transmit data to a UE that can receive the data via four transmission antennas.
Thus, in such an environment in which UEs each support different number of transmission antennas, a data transmission method with which the BS can satisfy the performance of all the UEs is required.